Creating sources of white light is the ultimate goal of solid state lighting technology. The largest market for LED-based lighting devices is the replacement of conventional incandescent and probably even fluorescent lamps. Practical white LED-based lighting devices became feasible only after the development of high brightness AlInGaN-based blue or near UV LEDs. Excited with short wavelength LEDs, white light emitting devices that exploit the mixture of two or three colors are being developed. In view of potential applications, the designs of these solid state light emitters aim at a combination of high efficiency and high color rendering.
There are basically two ways to produce white light emitting devices. The first approach is to mix light of different colors emitted by different chips. The other way is to down-convert the emission from a blue or UV LED to a longer wavelength light using phosphors. Where a blue LED is used, a part of the primary emission is used as a component of the white light emitting device as well. The number of phosphors involved in such a phosphor conversion (“PC”) LED may vary depending on the device characteristics required.
A straightforward way to devise a white light emitting device is to utilize an AlInGaN LED chip that emits blue light and a phosphor that emits in the yellow region. The blue chip is mounted in a built-in reflector cup and coated with a phosphor layer which is a mixture of encapsulant resin and phosphor powder. The entire structure is embedded in a transparent resin. A part of the blue light is absorbed in the phosphor layer and down converted to yellow light. The rest of the blue emission escapes into the transparent resin. A variety of phosphors for the down conversion of AlInGaN LED emission were considered. However, the most widely used phosphor is yttrium aluminum garnet (“YAG”) doped with trivalent cerium (Ce+). This phosphor emits a yellow light. A white light emitting device can be made by combining a blue emitting chip with this YAG phosphor.
Light emitting devices can also be constructed with metal silicate phosphors. There are several patents or patent applications that describe metal silicate phosphors and their application onto LED chips to make lighting emitting devices. WO 2004/067677 describes a strontium silicate-based phosphor (Sr2SiO4:Eu2+) combined with a blue LED chip to make a lamp. U.S. Pat. Nos. 7,294,956 and 7,023,019 describe a family of alkaline earth metal orthosilicate doped with divalent europium (Sr1-a1-b1-xBaa1Cab1Eux)SiO4 in orthorhombic crystal structure, and their use in combination with LED chip to construct lighting devices. U.S. Pat. No. 6,982,045 describes a similar formulation, SrxBayCazSiO4:Eu, and their application to LED devices. U.S. Pat. No. 7,267,787 describes a group of phosphor formula expressed by A2SiO4:Eu2+F where A is at least one of a divalent metal selected from the group consisting of Sr, Ca, Ba, Mg, Zn, and Cd, and wherein the fluorine dopant substitutes for oxygen and also described the utilization of the phosphors in LED devices. In a later patent, U.S. Pat. No. 7,311,858, and U.S. publication No. 2008-0073616, the formula A2SiO4:Eu2+D is described, where A is at least one of a divalent metal selected from the group consisting of Sr, Ca, Ba, Mg, Zn, and Cd; and D is a dopant selected from the group consisting of F, Cl, Br, I, P, S and N. In one embodiment described therein, the phosphor has the formula (Sr1-x-yBaxMy)2 SiO4:Eu2+F (where M is one of Ca, Mg, Zn, or Cd in an amount ranging from 0<y<0.5. A series of patents are granted to Toyoda Gosei, U.S. Pat. Nos. 7,157,746, 7,138,660, 6,943,380 and 7,227,190 in which the orthosilicate-based phosphor formulations are described for LED lighting applications. The general formula for the phosphors described therein is expressed by (Sr(1-x-y)BaxCay)2(Si(1-α-β-γ-δ)PαAlβBγGeδ)O4:Eu2+ where 0<x 0.8, 0y<0.8, 0<x+y<1, 0 α, β, γ<0.25, δ<0.5, 0 α+β+γ+<1.
There continues to be a need in the art for phosphors with enhanced emission efficiencies and improved stabilities that can be utilized in lighting technology. This need has been addressed by the present invention which is directed to a family of mixed metal silicate zirconate titanate hafnate phosphor formulations, doped with one or more metal ions, that can be excited with blue or near UV light and emit a yellow light.
All references described herein are incorporated by reference in their entireties for all purposes.